1. Field of the Invention
The present invention relates in general to methods of controlling the temperature of an exothermic reaction. More specifically, embodiments of the present invention are directed toward controlling the temperature of a Fischer-Tropsch synthesis by injecting a volatilizable liquid into the reactor, the volatilizable liquid being derived from the vapor products of the reaction.
2. State of the Art
It is generally desirable to remove at least a portion of the heat of reaction from a chemical reaction that is highly exothermic, such as a hydrocarbon synthesis. Excess heat of reaction may lead to an unacceptable increase in the temperature of the reactor contents, affecting the kinetics of the hydrocarbon synthesis. Changes in reaction kinetics may in turn adversely affect the molecular weight distribution of the products; for example, increasing the production of less desirable lower molecular weight products (e.g., methane), while simultaneously decreasing the production of the more valuable higher molecular weight products (e.g., hydrocarbons having a boiling point within the jet, naphtha, wax, and lubricant oil basestock range). If the reaction temperature is not properly controlled, the heat generated may exceed that for which the reactor was designed to handle, and in extreme situations, a serious safety hazard may develop.
Prior art methods of addressing heat generation from a highly exothermic reaction, such as a Fischer-Tropsch synthesis, have included the use of long, tubular-shaped reactors that have a greater surface area to volume ratio than more conventional cylindrical reactors, thereby providing additional surface area for cooling. Another method disclosed in the prior art has been directed to carrying out the reaction at low conversion rates such that unreacted gas passing through the reactor may be used to remove heat.
Process heat can be removed from exothermic reactors with indirect heat exchangers. Commonly, reactors are designed with internal heat exchangers equipped with cooling coils that are positioned inside the reactor. The cooling coils may be configured as structures other than coils, such as tubes, shells, fins, and the like. A cooling medium is circulated within the cooling coils such that the cooling medium is not in contact with the reaction medium. It is conventional to use water as a cooling medium, in which case the water may be converted to steam as the liquid water absorbs heat from the reactor contents. Alternatively, reactors may be designed with external heat exchangers. In each of these cases, the process stream or reaction medium is kept separated from the cooling medium because the cooling medium is confined within the walls of the heat exchanger""s cooling coils. When the cooling medium comprises liquid water, the temperature of the reactor may be regulated by controlling the pressure of the steam generated within the cooling coils. If higher temperatures are desired, the steam pressure within the coils may be set to a higher value. Conversely, when lower temperatures are desired, the coil steam pressure are set to a lower value.
The synthesis of hydrocarbons using a Fischer-Tropsch process is well known in the art. In the Fischer-Tropsch process, a mixture comprising substantially carbon monoxide (CO) and hydrogen (H2) is reacted over a catalyst to form hydrocarbon products having a broad spectrum of molecular weights ranging from methane (C1) to wax (C20+). Fischer-Tropsch processes which employ particulate fluidized beds and slurry bubble column reactors are also well known in the art. Slurry bubble column reactors operate by suspending catalytic particles in a liquid and feeding gas phase reactants into the reactor through a gas distributor which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they can be converted to both liquid and gaseous products. If gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered by passing the slurry through a filter which separates the liquid from the catalytic solids. A principal advantage of slurry reactors over fixed bed reactors is that the pressure of the circulating, agitated slurry phase increases the rate of heat transfer to cooling devices built into the reactor. An advantage of bubble column reactors is that the required mixing is affected by the action of rising bubbles, a process significantly more efficient than mechanical stirring.
While the Fischer-Tropsch reaction is catalyst dependent to a degree (i.e., dependent on the particular type and nature of the catalyst), its reaction rate generally increases with temperature. Employing a greater number of cooling coils within the reactor decreases the distance between adjacent coils and enhances temperature uniformity, but this comes at the expense of reactor volume that otherwise would have been available for catalyst loading, and conversion of Fischer-Tropsch reactants to products. A greater number of cooling coils also lowers the space velocity of the reactants. Thus, conventional prior art designs endeavor to balance these requirements when determining the appropriate number of cooling coils to use. On the other hand, the hottest spot in the reactor limits the temperature as it contributes to undesirable methane formation. Providing a more uniform temperature distribution throughout the reactor allows for operation at a higher average temperature without excessive methane formation.
Though the use of slurry bed reactors is advantageous for controlling the temperature of an exothermic reaction, there can be disadvantages. One drawback is the difficulty of separating wax products from fine catalyst particles, and the efficient conversion of reactants to products. The gas in the reactor can become depleted in one of the reactants, either hydrogen or carbon monoxide, for example, in which case the reaction rate will slow to below commercially viable levels. Another problem associated with a Fischer-Tropsch synthesis is that as reactant gases are converted into hydrocarbons and water, diluent gases in the feed stream gas, such as water vapor, light hydrocarbons, and contaminants, may dilute the hydrogen gas and the carbon monoxide gas to the point where the reaction rate of the exothermic reaction is significantly reduced.
Additionally, high temperatures often lead to carbon deposition on the catalyst and catalyst particle fragmentation. Carbon deposition and catalyst particle fragmentation is undesirable because the catalyst life is shortened.
Finally, since reactants in a Fischer-Tropsch reactor are typically gasses, the lack of uniform gas distribution within the reactor can affect reactor performance. Poor gas distribution can result in slug flow in slurry reactors or channeling in tubular reactors such that the reactor gases are not uniformly exposed to the catalyst. Gas maldistribution may result in a hot spot in the reactor which favors the undesired production of low molecular weight hydrocarbons, such as methane, as well as damage to the catalyst. The prior art teaches that such maldistribution and backmixing commonly occurs in conventional Fischer-Tropsch processes.
Thus, there is a need in the art for improved methods of heat removal from exothermic process reactors, including Fischer-Tropsch reactors, such that the desired yields of higher molecular weight hydrocarbons are produced. It is further desirable to have exothermic processes with sufficient temperature control such that catalyst deactivation through carbon deposition, and catalyst fragmentation is substantially avoided.
Embodiments of the present invention include methods of controlling the temperature of an exothermic reaction by:
a) contacting within a reactor a gaseous reactant with a catalyst to form reaction products, the reaction products existing in both a liquid and vapor phase;
b) removing at least a portion of the vapor phase reaction products from the reactor;
c) condensing at least a portion of the removed vapor phase reaction products at a location outside the reactor to form a volatilizable liquid; and
d) injecting at least a portion of the volatilizable liquid into the liquid phase reaction products contained within the reactor;
wherein the volatilizable liquid comprises at least 10 percent by weight C11+hydrocarbons.
In a particular embodiment, the volatilizable liquid comprises at least 10 percent by weight C11 to C20 hydrocarbons. In another embodiment, the volatilizable liquid comprises at least 10 percent by weight C11 to C15 hydrocarbons.
According to other embodiments, the exothermic reaction may be a Fischer-Tropsch synthesis, which, according to further embodiments, may be carried out in a slurry-type reactor. The present methods include the step of condensing at least a portion of the volatilizable liquid outside the reactor to remove from the reactor at least a portion of the heat generated by the exothermic reaction.
There are advantages in selecting hydrocarbons in the highest boiling point range of the materials exiting the reactor in the vapor phase for use (along with lower boiling hydrocarbons) as the volatilizable liquid. The higher boiling hydrocarbons include, in particular, C11+hydrocarbons. Under suitable operating conditions, this higher boiling material boils at a temperature that is substantially near the reaction temperature, and as a consequence, its removal from the reactor provides an effective method by which the temperature of the contents of reactor may be controlled. Since the boiling point of the higher boiling material is close to the reaction temperature, it is more effective in maintaining control of the reactor temperature at the desired set point than lighter components (which would continue to boil at undesirably low reaction temperatures).
In addition, the injection of the high boiling portion has the advantage of exerting a relatively small influence on the vapor pressure of the synthesis gas inside the reactor because the high boiling fraction contains the highest molecular weight materials of all the species in the vapor phase.
An additional benefit of adding the volatilizable liquid is that it will have a lower molecular weight than the slurry as a whole. This will in turn reduce the viscosity of the slurry. Reducing the viscosity improves both the mass and heat transfer, and an improved mass transfer will facilitate the conversion. Improved heat transfer at the catalyst surface will reduce the overheating of the catalyst by the exothermic reaction, which is desirable because a reduction in overheating will reduce methane yield. At cooling tube surfaces (if any cooling tubes are present within the reactor), lowering the viscosity of the slurry will also improve heat transfer. A higher boiling volatilizable liquid will be more effective than a lighter (lower boiling) one because a greater portion of the liquid will remain in the liquid phase for a longer period of time. Thus, a higher boiling volatilizable liquid will vaporize more slowly than the lower boiling one.